As “out with the Copper, in with the Fiber” gradually becomes a mainstream access manner of a network technology, and an optical access technology has been widely used. A passive optical network (PON) technology is an optical access technology based on point-to-multipoint (P2MP). A PON system mainly includes an optical line terminal (OLT) located in a central office and multiple optical network units (ONUs) located on a user side, where the OLT and the multiple ONUs are connected using an optical distribution network. A direction from the OLT to the ONU is defined as a downstream direction, and a direction from the ONU to the OLT is an upstream direction. The OLT sends a downstream optical signal to the multiple ONUs using the optical distribution network, and receives upstream optical signals from the ONUs. Correspondingly, each ONU receives a downstream optical signal from an OLT and sends an upstream optical signal to the OLT. In an upstream PON based on a time division multiple access manner, multiple ONUs send upstream optical signals through bursts (that is, send upstream burst optical signals), to reduce conflicts between the multiple ONUs.
An optical transmitter in a PON system mainly uses two modulation schemes, an external modulation and a direct modulation.
In an optical transmit system using external modulation, a laser and a modulator are independent, output light of the laser is injected into the modulator, and a bit stream (or an electrical drive signal) is used as a modulation signal to control the modulator such that a parameter of output light of the laser changes with the modulation signal. For example, the modulation signal may control the modulator such that an intensity or a phase of the output light of the laser changes with the modulation signal. Furthermore, an acousto-optic or electro-optic effect of the modulator may be used to control parameters of light. In an external modulation solution, because the laser works in a quiescent direct current state, an output optical signal has a small frequency chirp and high transmission performance. FIG. 1A shows a typical optical transmit system using external modulation, where a continuous wave distributed feedback (CW DFB) laser and an electro-absorption modulator form an electro-absorption modulated laser (EML). An optical signal output by the EML has a small frequency chirp and can well resolve a signal distortion problem caused by dispersion. However, the EML is high-cost, and causes a relatively large insertion loss (6 decibel (dB) to 8 dB) and also causes the insertion loss to remain high.
In an optical transmit system using direct modulation, an output optical signal of a direct modulated laser (DML) is changed by controlling a modulation current (that is, an injection current) of a DML. FIG. 1B shows a typical optical transmit system using direct modulation. A direct modulated distributed feedback (DM DFB) laser is one type of DML and is generally a semiconductor laser. The DM DFB laser has a simple structure, and is easy to implement and cost-effective.